1. Field of the Invention
The present invention relates to a transparent conductive layered structure comprising a transparent substrate and a transparent two-layered film being composed of a transparent conductive layer and a transparent coat layer being formed in succession on the transparent substrate, which is used in front panels, etc., of display devices such as Braun tubes (CRTs), plasma display panels (PDPs), vacuum fluorescent display (VFDs), liquid crystal displays (LCDs). The present invention particularly relates to a transparent conductive layered structure having the functions of preventing electrostatic charging or shielding an electric field, and preventing reflection, the two-layered film of which has an excellent scratch strength, and with which a reduction in production cost is expected and a method of producing the same, and a transparent coat layer forming coating liquid used in the method of producing the same, and a display device to which the transparent conductive layered structure is applied.
2. Description of the Related Art
Many OA devices have been introduced to the office as a result of office automation (OA) in recent years and an environment in which the entire day work must be done facing the display of OA device is no longer uncommon.
However, when a job is done next to a cathode ray tube (referred to as the above-described Braun tube: CRT) of a computer, etc., as an example of OA equipment, it must be easy to see the display screen in order to prevent visual fatigue, as well as prevent deposition of dust and electric shock induced by the electrostatic charge on the CRT screen, etc. Furthermore, in addition to these requirements, etc., there has recently been concern over the detrimental effects of low-frequency electromagnetic waves generated by CRTs on the human body and there is a demand for CRTs with which there is no leakage to the outside of such electromagnetic waves.
As measures for such prevention of electrostatic charging or prevention of leakage of electromagnetic waves (electric field shielding), there have been adopted conventionally a method of coating the front panel surface of a CRT, etc., with a transparent conductive layer, and other methods.
Additionally, it is desired that surface resistance of these transparent conductive layers be in the order of 107 to 1011 Ω/□ (ohm per square) for prevention of electrostatic charging, and be at least no more than 106 Ω/□ for prevention of leakage of electromagnetic waves, preferably no more than 5×103 Ω/□, and more preferably no more than 103 Ω/□. 
Therefore, several suggestions have been made thus far for meeting these requirements, but of these, the method wherein a coating liquid for forming a transparent conductive layer prepared by dispersing conductive microparticles in a solvent is applied to the front glass of a CRT and dried, and a coating liquid for forming a transparent coat layer comprising as its main component an inorganic binder such as silica sol, and then baked at a temperature of the order of 200° C. is known as a method with which low cost and low surf ac resistance can be realized.
This method that uses a coating liquid for forming a transparent conductive layer is very simple when compared to other methods of forming transparent conductive layers, such as vacuum evaporation and sputtering, has a low production cost, and is a very useful method.
Moreover, for the above-mentioned transparent microparticles, there are used transparent conductive oxide microparticles such as tin antimony oxide (ATO) or indium tin oxide or metal microparticles, etc. For example, in the case of application of ATO, because surface resistance of the film that is obtained is in the order of 107 to 1010 Ω/□, the film is used for preventing electrostatic charging.
In addition, although a coating liquid for forming a transparent conductive layer employing indium tin oxide is used for shielding an electric field, surface resistance of the film that is obtained is in the order of 104 to 106 Ω/□, which is insufficient to block leakage of an electric field, therefore a corrective circuit for canceling the electric field is needed.
On the other hand, when compared to coating liquids that use ITO, a film with somewhat lower transmittance, but also low resistance of 102 to 103 Ω/□, is obtained with coating liquids for forming transparent conductive layers that use metal microparticles for the above-mentioned conductive microparticles so that shielding an electric field can be performed without use of the above-mentioned corrective circuit.
Moreover, the metal microparticles that are used in the above-mentioned coating liquid for forming the above-mentioned transparent conductive layer are limited to noble metals, such as sliver, gold, platinum, rhodium, palladium, etc. that rarely oxidize in air, as shown in Japanese Patent Applications Laid-Open No. H 8-77832 and Laid-Open No. H 9-55175. This is because if microparticles of a metal other than a noble metal, such as iron, nickel, cobalt, etc., are used, an oxide film is invariably formed on the surface of such metal microparticles in an air atmosphere and good conductivity cannot be obtained as a transparent conductive layer.
Moreover, on the other hand, in order to make the display screen easy to see, anti-glare treatment is performed on the face panel surface to prevent reflection on the screen. This antiglare treatment is performed by the method whereby fine irregularities are made in the surface in order to increase diffused reflection at the surface, but it cannot be said that this method is a very desirable method because when used, resolution decreases and picture quality drops.
Consequently, it is preferred that antiglare treatment be performed by the interference method whereby the refractive index and film thickness of the transparent film be controlled so that there is destructive interference of the incident light by the reflected light.
A two-layered film structure wherein optical film thickness of film with a high refractive index and film with a low refractive index has been set at ¼λ and ¼λ, or ½λ and ¼λ, respectively, is usually used in order to obtain this type of low-reflection effect of the Interference method, and film consisting of the above-mentioned indium tin oxide (ITO) microparticles is also used as this type of film with a high refractive index.
Furthermore, of the optical constant (n−ik, n: refractive index, i2=−1, k: extinction coefficient) of metals, the value of n is small, but the value of k is very large when compared to ITO and therefore, even if a transparent conductive layer consisting of metal microparticles is used, the same anti-reflection activity induced by interference of light as seen with ITO is obtained with the two-layered film structure.
Moreover, in recent years, in addition to the properties such as the above-mentioned good conductivity, low reflectance, etc., this type of transparent conductive layered structure is requested to have the property of improving the contrast of images by adjusting the transmittance to the predetermined range (40 to 75%) lower than 100% in order to make the display screen easier to see. In this case, the mixing of color pigment microparticles, etc., into the above-mentioned coating liquid for forming a transparent conductive layer also has been performed.
In addition to this, as treatment for water resistance, brine resistance or anti-smudge of these transparent conductive layers, several attempts to make the layers water-repellent have been carried out recently.
Such a transparent conductive layered structure having a transparent two-layered film being composed of a transparent conductive layer and a transparent coat layer is produced, as mentioned above, by applying a coating liquid for forming a transparent conductive layer on a transparent substrate, and drying, and applying a coating liquid for a transparent coat layer as its main components silica sol, etc., and then conducting a heat treatment at a temperature of the order of 200° C.
Moreover, because the above-mentioned transparent coat layer comprises as its main component silicon oxide, the layer is formed as a film of relatively high strength by heat treatment at a temperature of the order of 200° C.
However, depending on the producers or makers of CRTs, there are cases where the above-mentioned heat treatment temperature is performed only under the condition of the order of 160° C. because of the limitation of facility aspect, etc., so that the strength of some transparent coating films may be insufficient due to the low temperature heating.
Similarly, for a coating liquid for forming a transparent conductive layer using the above-mentioned noble metal microparticles, the coating strength of some transparent two-layered films that are obtained is insufficient. It is considered that this is because when compared to conductive oxide microparticles such as ITO, etc. noble metal microparticles are chemically inert, and thus the binding between noble metal microparticles and an inorganic binder matrix is not so strong, and therefore the coating strength of the transparent two-layered films decreases.